Our objective is to define in genetic and molecular terms the mechanisms which regulate the expression of genetic information during cell growth and development. For this purpose, we are investigating two model systems which serve as analogs for erythrodifferentiation and immunodifferentiation. This work has led to the development of techniques for gene isolation and the proof of certain hypotheses related to the expression and the origin of immunoglobulin genes. During the past year, special emphasis has been given to the development of safe vectors for use in the cloning of recombinant DNA molecules from higher organisms. Two such safe vector systems involving the use of the bacteriophage lambda have been developed and extensively tested. As a result, the first so-called EK2 biologic containment system has been certified by the NIH Committee on Recombinant DNA Research. Additional work has been directed towards the specific purification and cloning of specific mouse genes, in particular those responsible for the synthesis of mouse ribosomal RNA. BIBLIOGRAPHIC REFERENCES: Honjo, T., Swan, D., Nau, M., Norman, B., Packman, S., Polsky, F. and Leder, P.: Purification and Translation of an Immunoglobulin lambda Chain Messenger RNA from Mouse Myeloma. Biochemistry 15: 2775-2779, 1976. Leder, P., Tiemeier, D. and Enquist, L.: EK2 Derivatives of Bacteriophage lambda Useful in the Cloning of DNA from Higher Organisms: The lambda gtWES System. Science 196: 175-177, 1977.